Metal complexes are well known for their chemical or physicochemical properties and are exploited in this respect in many industrial fields including, for example, the fields of catalysis, magnetism, molecular electronics, luminescence, the environment, imaging and therapeutics.
In general, these complexes involve one or more metals coordinated with one or more ligands.
Among the potential ligands, tetradentate ligands of diimine-dioxime type are particularly appreciated due to their ability to form stable complexes with transition metals such as rhodium (Rh), iron (Fe), silver (Ag), copper (Cu), nickel (Ni) or cobalt (Co) (cf. Collman, J. P., Brauman, J. I., Madonik, A. M., Organometallics, 1986, 5, 310-322, Anderson, O. P., Perkins, C., Brito, K., Inorg. Chem. 1983, 22, 1267-1273, Eltayeb, M. A., Sulfab, Y., Polyhedron, 2007, 26, 39-42, Stynes, Dennis V.; Singh, Kowsill; Ng, Betty; Wilshire, Susan; Inorg, Chim. Acta; Vol. 58; 1982, 58, 179-186). In general, a diimine-dioxime ligand coordinates the metal atoms by adopting a planar or quasi-planar conformation. It is then said to coordinate the metal in an equatorial plane. Other coordination positions are then left vacant, and are termed “axial coordination positions”.
In particular, it is known practice to use certain diimine-dioxime complexes as catalysts for polymerization reactions of methyl methacrylate (cf. Zangrando, E.; Trani, M.; Stabon, E.; Carfagna, C.; Milani, B.; Mestroni, G. Eur. J. Inorg. Chem. 2003, 2683-2692) or for electroproduction of hydrogen (cf. Jacques. P.-A.; Artero, V.; Pécaut, J.; Fontecave, M.; Proc. Natl. Acad. Sci. USA. 2009, 116, 20627-20632).
Such complexes may also be used in imaging, in which a 64Cu diimine-dioxime complex has been envisaged as a radiopharmaceutical compound for positron emission tomography (cf. Kiani, S.; Staples, R. J.; Treves, S. T.; Packard, A. B. Polyhedron 2009, 28, 775-781).
It is also known practice to use cobalt diimine-dioxime complexes as vitamin B12 mimics. These complexes are capable of stabilizing either a cobalt-hydride bond, or a cobalt-carbon bond that can undergo homolytic cleavage (cf. Gerli, A.; Sabat, M.; Marzilli, L. G.; J. Am. Chem. Soc. 1992, 114, 6711-6718).
The immobilization of one or more complexes on a surface offers many advantages. Specifically, in the case where the immobilized chemical species is a catalyst, its immobilization makes the purification of the final products and the recycling of the catalyst easier by mechanical separation of the immobilized catalyst from the reaction medium. In other cases, immobilization can render a catalyst more stable and thus increase its lifetime and its performance. In a context of electrocatalysis, the immobilization of a catalyst on an electrode can also optimize the flow of electrons from the electrode to the catalyst and thus improve the current densities.
Other applications may moreover require the coupling of a ligand or a complex with another molecule, in order to be able to combine the properties of the two partners. In this respect, mention may be made, for example, of the combination of a polydentate ligand with a fluorescent species. In this case, complexation of the ligand modifies the fluorescence properties of the system, which makes it possible to obtain sensors that give information regarding the presence of metal ions in the medium (cf. Valeur, B.; Leray, I. Coord. Chem. Rev. 2000, 205, 3-40).
The diimine-dioxime ligands that have been the most extensively studied are those with a carbon chain containing two or three atoms between the two nitrogen atoms of the imine functions. This spacing leads, after complexation, to stable 5- or 6-membered metallocycles. As illustrations of ligands of this type, mention may be made of those described in the publication by Dey et al; Synth, React. Inorg. Met.-Org. Chem.; 2004, 34; 1615-1634, which bear an occasionally protected hydroxyl function.
Complexes involving these ligands can theoretically lend themselves to covalent coupling with a substrate in several orientations, namely: the axial orientation (i.e. perpendicular to the plane of the ligand) and the equatorial orientation (i.e. in the plane of the ligand). More particularly, the coupling may take place via the diimine-dioxime ligand, and this is then referred to as coupling in the equatorial orientation (i.e. in the plane of the ligand). Alternatively, the coupling may be performed via an axial ligand, and this is then referred to as coupling in the axial orientation (i.e. generally perpendicular to the plane of the ligand).
The inventors have found that, in comparison with immobilization in the axial position via a generally monodentate ligand, the immobilization mode in the equatorial orientation is stabler since the polydentate nature of the diimine-dioxime ligand makes it possible to reduce the probability of detachment of the metal complex.
However, the hydroxyl functions of the abovementioned diimine-dioxime ligands, which are precisely suitable for coupling in the equatorial orientation, in fact prove in practice to be unsuitable for covalent coupling.
Specifically, the formation of an ether via the Williamson reaction, requiring the production of the corresponding alkoxide, is performed under very basic conditions which the diimine-dioxime ligand residue does not withstand. Coupling by esterification is also forbidden due to the saponification equilibrium that draws into doubt the stability of the final structure. Furthermore, the chemistry on this hydroxyl function comes into competition with the hydroxyl functions borne by the oxime functions, which must maintain their integrity so as not to modify the first coordination sphere of the metal complex.
Thus, there is a need for diimine-dioxime ligands or for corresponding metal complexes that can be immobilized stably over time on a surface of a substrate.
There is also a need to prepare molecular assemblies by covalent coupling of diimine-dioxime ligands or of metal complexes thereof with other molecules.
The present invention is directed towards solving all or some of the abovementioned needs.